The present invention relates to gravure printing, and more specifically to a method and apparatus for forming gravure cells in a gravure cylinder.
Presently, with raster or screen gravure printing, copper cylinders are provided with a number of gravure cells of specified diameter and depth each of which corresponds to the tonal intensity of a particular image point of the original copy to be reproduced. The gravure cells are generally produced by chemical etching or by mechanical engraving using a stylus or by laser energy.
In the chemical etching process, formation of the individual gravure cells corresponding to the original copy is controlled by means of a variable permeability negative mask which covers the surface of the printing form. Permeability to the etchant is varied in accordance with the tonal values of the image points of the original copy. The mask (carbon tissue resist or DuPont Rotofilm resist) is formed by a photographic process of exposure to continuous tone and halftone positives made from the original copy. After transfer of the mask to the cylinder and development, the action of the etchant during the etching process occurs gradually over the surface of the gravure cylinder depending upon the level of light exposure at different points on the carbon tissue or Rotofilm resist. This method of providing a production gravure cylinder uses messy chemicals, is time-consuming, and highly operator dependent due to changing parameters such as the concentration of the etchant.
Presently, mechanical engraving is done with the Helio-Klischograph system available from the Hell Company or similar devices. Image information is either optically scanned from suitably prepared photographic copy or taken from computer memory. This information is used to direct a diamond stylus, vibrating it back and forth as the gravure cylinder is rotated. The diamond stylus cuts cells into the gravure cylinder in accordance with desired tones. This method of providing a production gravure cylinder is limited by the shape of the stylus and is often followed by etching to provide deeper cells.
U.S. Pat. No. 4,108,659 filed in the name of Mamilina Dini, discloses a method of engraving gravure cylinders with a laser beam which eliminates point-by-point scanning of the original picture format and therefore modulation of the energy output of the laser beam. The engraving power of the laser beam is controlled by a mask which modulates the laser beam in accordance with the tonal gradations in the original picture format. That is, the mask is constructed so that its reflectivity is inversely proportional to the tonal gradations of the original picture format. Engraving in accordance with this process requires the precise formation of the mask to accurately produce the desired pattern of gravure cells having the proper diameter and depths.
Another technique utilizing lasers includes coating the gravure cylinder with an epoxy material which is absorbent to a high power laser beam. The laser beam is used to produce a groove of varying depth and width rather than individual gravure cells. The lasers employed are quite expensive due to the high power levels required, as well as large in size. Further, special preparations are required for baking the epoxy coated cylinder. Additionally, there are uncertainties regarding the wear resistance of the epoxy material.